Bayonet Neill Concelman (BNC) coaxial connectors are commonly-used in the electronics industry for coupling low voltage signals to a measurement instrument, such as an oscilloscope. The mating halves of BNC connectors are referred to as male and female, respectively. Typically, the male portion of the BNC connector is mounted to the instrument chassis, and the female portion is attached to the end of a coaxial cable, such as a probe cable. The cable-mounted female potion of the BNC connector is secured to the panel-mounted male portion by way of a pair of metal “bayonets” mounted on the male portion of the BNC connector perpendicularly to the major axis of the connector.
In ground-referenced oscilloscopes, the outer shell of the Panel-mounted BNC connector is physically and electrically connected to the chassis of the oscilloscope. The oscilloscope chassis, in turn, is coupled to earth ground through the ground wire of its power cord. So long as no effort is made to interfere with this grounding system, it is safe for an operator of the oscilloscope to touch the outer shell of the BNC connector.
When a probe is connected to the grounded BNC connector on the oscilloscope, the metallic shield of the probe's cable is coupled to ground potential. The ground clip of the probe is also connected to the coaxial shield. Thus, all measurements being taken will be referenced to ground potential. However, there are times when it is necessary for an engineer to make a measurement that is not ground-referenced. In such circumstances, some engineers have resorted to a not-recommended and potentially dangerous practice known as “floating the scope”. The term “floating the scope” refers to the above-mentioned practice of interfering with the grounding system of the oscilloscope by breaking the connection between the oscilloscope and earth ground. In such a case, the oscilloscope chassis and the shell of the BNC connector may develop a high and potentially lethal voltage on exposed surfaces.
Fortunately, there is a better solution to the problem of making non-ground-referenced measurements than “floating the scope”. The THS700-series portable oscilloscopes manufactured by Tektronix, Inc., Beaverton, Oreg., employ a non-conductive case and a BNC connector having a non-conductive outer surface to prevent inadvertent contact of the user with dangerous voltages that may otherwise be present.
However, there is a problem that arises when such a solution is used. Safety requirements for an isolated instrument require the use of non-metallic BNC shells. The non-metallic shells necessarily require a pair of non-metallic bayonets. These have a history of failing because the bayonets are prone to being sheared from the shell. That is, the forces applied to the BNC connector when mating it will eventually cause the insulating material of the bayonets to fail. When this happens, the oscilloscope must be returned for service. In fact, the rate of replacement of damaged non-metallic BNC connectors can run as high as 300 BNC's per year for this exact problem. Without the bayonets, the mating connector will not be securely coupled to the Panel-mounted portion, and will simply fall off under normal use conditions.
One might think that a solution to the problem would be to change the size or shape of the bayonets to increase their strength. Unfortunately, geometry changes to increase strength are not permitted because the instrument must remain compatible with all existing BNC receptacles owned by the customer.